As one of surface analysis methods performed by an infrared microscope, there is an attenuated total reflection (ATR) method (hereinafter, a measurement by the ATR method is referred to as “ATR measurement). In the ATR measurement, as shown in FIG. 13A, a sample S is press-contacted to a prism (ATR prism) 438 having a higher refractive index than that of the sample S, and infrared light is irradiated on the surface of the sample S at an incident angle equal to or more than the total reflection critical angle. Then, after incident on the ATR prism 438, the infrared light is totally reflected by the boundary surface B between the ATR prism 438 and the sample S. At the time of the total reflection, as shown in FIG. 13B, the infrared light slightly (a fraction of the wavelength of the measured infrared light) penetrates toward the sample S side beyond the boundary surface B and receives inherent absorption by the surface portion of the sample S. As described above, in the ATR measurement, the sample surface analysis can be performed by analyzing the absorption spectrum of the infrared light slightly penetrated into the sample surface and then reflected from the sample.
FIG. 14 is a diagram showing a configuration of a general objective optical system used in an infrared microscope in recent years. The illustrated objective optical system includes Cassegrain mirrors (also called Schwarzschild type reflection objective mirrors), an approximately hemispherical ATR prism 538, and a prism holder 537 for holding the ATR prism 538. The Cassegrain mirrors are provided with a concave primary mirror 511 having an opening 513 in the center and a convex secondary mirror 512 arranged immediately below the opening 513. The primary mirror 511 is fixed with the concave surface facing downward, and the secondary mirror 512 is fixed with the convex surface facing upward. The diameter of the ATR prism 538 is about several millimeters. Here, the bottom surface of the ATR prism 538 is a perfect plane or a spherical shape bulging slightly downward, and the region where the ATR prism 538 and the sample S are in contact is a small region of about several tens to several hundreds of micrometers in diameter. Hereinafter, this small region will be referred to as “contact point P” between the prism and the sample. In addition to the above, an infrared light source, a visible light source, a detection optical system for detecting infrared light, a visual optical system for visually observing a sample using visible light, a sample stage 580 for arranged a sample S, etc., are included as constituent elements of the infrared microscope.
The light (measurement light) from the infrared light source is incident on the secondary mirror 512 from above the objective optical system via the opening 513, and is reflected by the convex surface of the secondary mirror 512 and incident on the primary mirror 511. The measurement light reflected and condensed by the concave surface of the primary mirror 511 is incident on the ATR prism 538 arranged at the focal point of the primary mirror 511 and irradiated to the contact point P. The reflected infrared light from the sample S is incident on a detection optical system of the infrared microscope through the primary mirror 511 and the secondary mirror 512 and detected.
As described above, the ATR method is an analysis method in which a total reflected light absorbed and attenuated in the process that the measurement light slightly penetrated the sample surface passes through the sample surface layer is measured to obtain an absorption spectrum of the sample surface layer. The penetration depth of the measurement light at this time depends on the refractive index “n” of the ATR prism and the incident angle θ of the light to the sample. Among these, in order to change the refractive index “n”, it is necessary to prepare a plurality of ATR prisms of different materials. However, an ATR prism is relatively expensive, resulting in an increased cost for the ATR measurement.
Also, in order to obtain high optical throughput by the ATR measurement, it is necessary to widen the incident angle range of the light incident on the sample from the Cassegrain mirror. When the minimum incident angle is decreased to increase the solid angle of the opening of the reflection objective mirror, the minimum incident angle approaches the critical angle. Therefore, due to the influence of anomalous dispersion of refractive index “n”, there is a problem that shape changes of the absorption peak (differential shape formation, low wave number peak intensity becomes relatively large, shift to a low wave number side, etc.) occurs easily. On the other hand, in order to suppress the influence of anomalous dispersion of refractive index “n”, it is necessary to increase the minimum incident angle of the light incident on the sample from the Cassegrain mirror. However, in that case, there is a problem that the solid angle of the opening of the Cassegrain mirror decreases and therefore the optical throughput decreases. As described above, there is a trade-off relationship between the improvement of optical throughput and the reduction of anomalous dispersion, and in a conventional objective optical system, the angle range of incident light is fixed according to the measurement purpose and the design intention. Therefore, when performing a measurement which prioritizes the optical throughput and a measurement which prioritizes mitigation of the anomalous dispersion of refractive index “n”, respectively, it is necessary to select one of a plurality of objective optical systems.
In order to solve these problems, Patent Document 1 describes an objective optical system that can obtain different absorption spectra different in penetration depth by using a single ATR prism by changing the incident angle range of light to the sample. As shown in FIG. 15A, in the objective optical system, a light shielding mask M having an arcuate opening is arranged above the secondary mirror 612 so that a part of the measurement light incident on the secondary mirror 612 from the infrared light source can be shielded. A plurality of light shielding masks M different in shape and size of the openings are prepared. By switching light shielding masks to be placed on the optical path of the measurement light by a predetermined slide mechanism, it is possible to change the incident angle range of the light incident on the sample S via the secondary mirror 612, the primary mirror 611, and the ATR prism 638.